1. Field of the Invention
This invention relates to MOS transistors and in particular to an MOS silicon transistor wherein the gate oxide is formed so as to achieve extremely stable and reproducible MOS transistors with perdictable characteristics.
2. Prior Art
Mos semiconductor transistors are well known. Such transistors are extremely sensitive to small amounts of contamination at the interface between insulation layers and the underlying semiconductor material containing the source and drain regions. As the size of MOS transistors has decreased, proper alignment of masks and particularly the source and drain masks, has become more important. The use of a self-aligned gate of polycrystalline silicon as disclosed in Klein et al U.S. Pat. No. 3,673,471 issued June 27, 1972 makes possible the reduction in size of the source and drain regions, reduces the overlap of the gate with respect to the source and drain regions and thus makes possible higher speed performance of MOS transistors.
In an MOS transistor, a thin insulating layer is placed between the semiconductor substrate containing the source and drain regions and the gate electrode. To prevent unwanted inversions of the semiconductor material in the field (i.e., the non-active portion) of the device when a voltage is applied to the gate electrode, a much thicker insulating layer is placed over the field of the device than under the gate electrode. As disclosed in the above-mentioned Klein et al patent, typically the field insulation is an order of magnitude thicker than the gate insulation. To make an MOS transistor by the prior art methods, the field oxide is first formed on the wafer. Those portions of the field oxide over the regions of the semiconductor substrate in which sources and drains are to be formed are then removed. After formation of the source and drain regions, the field oxide over the gate region is removed and the gate oxide if formed. The gate oxide is typically of a thickness on the order of 1,000 angstroms.
The removal of the field oxide over the active regions of the semiconductor substrate allows these regions of the substrate to become contaminated and makes difficult the further processing of the device to grow a uniform thickness gate oxide. Typically, contaminants gather on the edges of the field oxide and result in short circuit forming between a subsequently formed gate electrode and the source and/or drain regions. In addition, the different thickness of the field and gate oxides cause an abrupt step in the insulation adjacent the source and drain regions. Such a step greatly increases the risk of open circuits in the conductive leads contacting the source and drain regions.